Britain rushes into a revenue grab not even Brussels has tried.

It’s a bad sign for Britain that what most global investors will notice about the government’s new budget is its worst idea. Chancellor Philip Hammond on Monday used his last budget before Brexit to unveil a first-in-the-world Digital Services Tax that’s a headscratcher on political and economic grounds.

Call this DST what it is: a raid on American tech giants. Mr. Hammond plans to raise £1.5 billion ($1.9 billion) over four years by imposing a 2%-of-revenues tax on the earnings of specified tech companies within the U.K....